Composite structure type high tensile strength steel plate, plated plate of composite structure type high tensile strength steel and method for their production

ABSTRACT

The invention proposes a high-strength dual-phase cold rolled steel sheet having an excellent deep drawability, wherein the steel sheet has a composition comprising C: 0.01-0.08 mass %, Si: not more than 2.0 mass %, Mn: not more than 3.0 mass %, P: not more than 0.10 mass %, S: not more than 0.02 mass %, A1: 0.005-0.20 mass %, N: not more than 0.02 mass % and V: 0.01-0.5 mass %, provided that V and C satisfy a relationship of 0.5×C/12≦V/51≦3×C/12, and the remainder being Fe and inevitable impurities, and has a microstructure consisting of a ferrite phase as a primary phase and a secondary phase including martensite phase at an area ratio of not less than 1% to a whole of the microstructure and a high-strength dual-phase galvanized steel sheet comprising a galvanized coating on the above steel sheet as well as a method of producing the same.

TECHNICAL FIELD

[0001] This invention relates to a high-strength dual-phase steel sheethaving an excellent deep drawability, and particularly to ahigh-strength dual-phase cold rolled steel sheet having an excellentdeep drawability and a high strength dual phase galvanized steel sheethaving an excellent deep drawability which have a tensile strength of440 MPa or more and are suitable for use in steel sheets for vehicles aswell as a method of producing the same.

BACKGROUND ART

[0002] Recently, it is required to improve a fuel consumption in avehicle from a viewpoint of the maintenance of the global environment,and also it is required to improve a safety of a vehicle body from aviewpoint of the protection of crews during the collision of thevehicle. To this end, investigations for achieving both the lighteningand strengthening of the vehicle body are positively proceeding.

[0003] In order to simultaneously satisfy the lightening andstrengthening of the vehicle body, it is said that thehigh-strengthening of raw materials constituting the parts is effective,and recently, high-strength steel sheets are positively used as a partof the vehicle.

[0004] Most of the parts for the vehicle body are formed by pressworking of the steel sheet as a raw material. To this end, thehigh-strength steel sheet used is required to have an excellent pressformability. In order to improve the press formability, it is necessaryto have a high Lankford value (r-value), a high ductility (E1) and a lowyield stress (YS) as mechanical properties of the steel sheet.

[0005] However, in general, as the steel sheet becomes highlystrengthened, the r-value and the ductility lower and the pressformability is degraded, while the yield stress rises to degrade theshapability and hence the problem of springback is apt to occur.

[0006] And also, a high corrosion resistance is required according to aposition of the vehicle part to be applied, so that varioussurface-treated steel sheets having an excellent corrosion resistanceare used as a steel sheet for the vehicle parts up to now. Among thesesurface-treated steel sheets, a galvanized steel sheet is manufacturedin a continuous galvanizing equipment conducting recrystallizationannealing and galvanizing at the same line, so that the provision of anexcellent corrosion resistance and a cheap production are possible. Andalso, an alloyed galvanized steel sheet obtained by subjecting to a heattreatment after the galvanization is excellent in the weldability andpress formability in addition to the excellent corrosion resistance.Therefore, they are widely used.

[0007] In order to further advance the lightening and strengthening ofthe vehicle body, in addition to the development of the high-strengthcold rolled steel sheet having the excellent press formability, it isdesired to develop a high-strength galvanized steel sheet having anexcellent corrosion resistance through the continuous galvanizing line.

[0008] As a typical example of the high-strength steel sheet having agood press formability is mentioned a dual-phase steel sheet having adual-phase microstructure of a soft ferrite phase and a hard martensitephase. Especially, the dual-phase steel sheet produced by cooling with agas jet after the continuous annealing is low in the yield stress andpossesses a high ductility and an excellent baking hardenability. Theabove dual-phase steel sheet is generally good in the workability, buthas a drawback that the workability under severer condition is poor andparticularly, the r-value is low and the deep drawability is bad.

[0009] And also, when the galvanization is applied for providing theexcellent corrosion resistance, the continuous galvanizing line isgeneral to set up the annealing equipment and the plating equipmentcontinuously. To this end, in case of subjecting to the galvanization,the cooling after the annealing is constrained by a plating temperatureand can not drop down to a temperature lower than the platingtemperature at once and hence the cooling is interrupted. At a result,an average cooling rate necessarily becomes smaller. Therefore, when thegalvanized steel sheet is produced in the continuous galvanizing line,it is difficult to generate martensite phase produced under a coolingcondition of a large cooling rate into the steel sheet after thegalvanization. To this end, it is generally difficult to produce thehigh-strength galvanized steel sheet having a dual-phase microstructureof a ferrite phase and a martensite phase through the continuousgalvanizing line.

[0010] Under such unfavorable conditions, it is attempted to increasethe r-value of the dual-phase steel sheet to improve the deepdrawability. For example, JP-B-55-10650 discloses a technique that a boxannealing is carried out at a temperature ranging from arecrystallization temperature to A_(c3) transformation point after thecold rolling and thereafter the continuous annealing inclusive ofquenching and tempering is carried out after the heating to 700-800° C.in order to obtain the mixed microstructure. In this method, however,the quenching and tempering are carried out during the continuousannealing, so that the yield stress is high and hence a low yield ratiocan not be obtained. The steel sheet having such a high yield stress isnot suitable for the press formability and has a drawback that theshapability in the pressed parts is bad.

[0011] And also, a method for lowering the high yield stress isdisclosed in JP-A-55-100934. In this method, the box annealing is firstcarried out in order to obtain a high r-value, wherein the temperaturein the box annealing is made to a two-phase region of ferrite(α)-austenite (γ) and Mn is enriched from α phase to γ phase during thesoaking. As the Mn enriched phase preferentially becomes γ phase duringthe continuous annealing, the dual-phase microstructure is obtained evenat a cooling rate as in the gas jet cooling, and further the yieldstress becomes low. In this method, however, it is required to conductthe box annealing at a relatively high temperature being the α-γtwo-phase region over a long time for enriching Mn, so that there aremany problems in production steps such as a frequent occurrence ofadhesion between steel sheets inside a coil resulted from the thermalexpansion in the annealing, an occurrence of temper color, a lowering ofservice life in an inner cover for a furnace body and the like.Therefore, it was difficult to industrially stably produce high-strengthsteel sheets possessing a high r-value and a low yield stress up to now.

[0012] In addition, JP-B-1-35900 discloses a technique wherein thedual-phase cold rolled steel sheet having a very high r-value and a lowyield stress of r-value=1.61, YS=224 MPa and TS=482 MPa can be producedby cold rolling a steel having a composition of 0.012 mass % C-0.32 mass% Si-0.53 mass % Mn-0.03 mass % P-0.051 mass % Ti, heating to 870° C.corresponding to α-γ two-phase region and thereafter cooling at anaverage cooling rate of 100° C./s. However, the high cooling rate of100° C./s is difficult to attain in the gas jet cooling usually used inthe continuous annealing line or continuous galvanizing line after thecold rolling, and is required to use an equipment for water-quenching,and also a problem becomes actual in the surface treatment of thewater-quenched steel sheet, so that there are problems in the productionequipment and the materials.

[0013] Furthermore, it is attempted to produce the high-strengthdual-phase galvanized steel sheet. In the past, as the method ofproducing the high-strength dual-phase galvanized steel sheet isgenerally used a method wherein the formation of low-temperaturetransformation phase is facilitated by using a steel added with a largeamount of an alloying element such as Cr or Mo for enhancing ahardenability. However, the addition of the large amount of the alloyingelement undesirably brings about the rise of the production cost.

[0014] Moreover, as is disclosed in JP-B-62-40405 and the like, there isproposed a method of producing the high-strength dual-phase galvanizedsteel sheet by defining the cooling rate after the annealing or theplating in the continuous galvanizing line. However, this method is notactual from the constraint on the equipment for the continuousgalvanizing line and also the steel sheet obtained by this method is notsaid to have a sufficient ductility.

DISCLOSURE OF THE INVENTION

[0015] It is, therefore, an object of the invention to solve theaforementioned problems and to provide high-strength dual-phase coldrolled steel sheets having an excellent deep drawability andhigh-strength dual-phase galvanized steel sheets having an excellentdeep drawability as well as a method of producing the same.

[0016] Moreover, the term “galvanized steel sheet” used herein means toinclude a galvanized steel sheet obtained by subjecting to agalvanization containing aluminum or the like in addition to zinc and analloyed galvanized steel sheet obtained by subjecting to a heat(alloying) treatment for diffusing iron of the matrix steel sheet intothe plated layer after the galvanization.

[0017] In order to achieve the above object, the inventors have madevarious studies with respect to an influence of the alloying elementupon the microstructure and the recrystallization texture in the steelsheet. As a result, it has been found that by limiting C in a steel slabto a lower content and rationalizing V content in relation to C content,before the recrystallization annealing, C in the steel is precipitatedas a V carbide to decrease solid-solute C as far as possible to therebydevelop {111} recrystallization texture to obtain a high r-value andsubsequently the V carbide is dissolved by heating to α-γ two-phaseregion to enrich C in austenite for easily generating martensite in asubsequent cooling process, whereby the high-strength dual-phase coldrolled steel sheet and high-strength dual-phase galvanized steel sheethaving a high r-value and an excellent deep drawability can be producedstably.

[0018] The results of fundamental experiments performed by the inventorswill be explained below.

[0019] In this case, the experiments are performed with respect to ahigh-strength dual-phase cold rolled steel sheet of TS: 590 MPa gradeand a high-strength dual-phase cold rolled steel sheet of TS: 780 MPagrade.

[0020] Firstly, the fundamental experiment in the high-strengthdual-phase cold rolled steel sheet of TS: 590 MPa grade is performedunder the following conditions. Each of various sheet bars having abasic composition of C: 0.03 mass %, Si: 0.02 mass %, Mn: 1.7 mass %, P:0.01 mass %, S: 0.005 mass %, Al: 0.04 mass % and N: 0.002 mass % anddifferent V contents by adding V within a range of 0.03-0.55 mass % isheated to 1250° C. and soaked, and then subjected to three-pass rollingat a finisher delivery temperature of 900° C. to obtain a hot rolledsteel sheet having a thickness of 4.0 mm.

[0021] In addition, the same procedure as described above is conductedwith respect to various sheet bars having a basic composition of C: 0.03mass %, Si: 0.02 mass %, Mn: 1.7 mass %, P: 0.01 mass %, S: 0.005 mass%, Al: 0.04 mass % and N: 0.002 mass % and different values of (2×Nb[mass %]/93+2×Ti [mass %]/48)/(V [mass %]/51) by adding V, Nb and Tiwithin ranges of 0.03-0.04 mass %, 0.01-0.18 mass % and 0.01-0.18 mass%, respectively, so as to satisfy a relationship of 0.5×C [mass %]/12≦(V[mass %]/51+2×Nb [mass %]/93+2×Ti [mass %]/48)≦3×C [mass %]/12.

[0022] Moreover, the hot rolled steel sheet after the finish rolling issubjected to a temperature holding treatment of 650° C.×1 hour as acoiling treatment. Subsequently, the sheet is subjected to a coldrolling at a rolling reduction of 70% to obtain a cold rolled steelsheet having a thickness of 1.2 mm. Next, the cold rolled steel sheet issubjected to a recrystallization annealing at 850° C. for 60 seconds andcooled at a cooling rate of 30° C./s.

[0023] On the other hand, the fundamental experiment in thehigh-strength dual-phase cold rolled steel sheet of TS:780 MPa grade isperformed under the following conditions.

[0024] Each of various sheet bars having a basic composition of C: 0.04mass %, Si: 0.70 mass %, Mn: 2.6 mass %, P: 0.04 mass %, S: 0.005 mass%, Al: 0.04 mass % and N: 0.002 mass % and different values of(2×Nb/93+2×Ti/48)/(V/51) by adding V, Nb and Ti within ranges of0.02-0.06 mass %, 0.01-0.12 mass % and 0.01-0.12 mass %, respectively,so as to satisfy a relationship of 0.5×C [mass %]/12≦(V [mass %]/51+2×Nb[mass %]/93+2×Ti [mass %]/48)≦3×C [mass %]/12is heated to 1250° C. andsoaked, and then subjected to three-pass rolling at a finisher deliverytemperature of 900° C. to obtain a hot rolled steel sheet having athickness of 4.0 mm. Moreover, the sheet after the finish rolling issubjected to a temperature holding treatment of 650° C.×1 hour as acoiling treatment. Subsequently, the sheet is subjected to a coldrolling at a rolling reduction of 70% to obtain a cold rolled steelsheet having a thickness of 1.2 mm. Next, the cold rolled steel sheet issubjected to a recrystallization annealing at 850° C. for 60 seconds andcooled at a cooling rate of 30° C./s.

[0025] With respect to the thus obtained cold rolled steel sheets isconducted out a tensile test to investigate tensile properties. Thetensile test is carried out by using JIS No. 5 tensile test piece. Ther-value is determined as an average r-value {=(r_(L)+_(C)+2×r_(D))/4} ina rolling direction (r_(L)), a direction (r_(D)) inclined at 45 degreewith respect to the rolling direction and a direction (r_(C))perpendicular (90°) to the rolling direction.

[0026]FIGS. 1a and 1 b show an influence of V content in a steel slabupon r-value and yield ratio of a cold rolled steel sheet (YR=yieldstress (YS)/tensile strength (TS)×100(%)) in cold rolled steel sheets ofTS: 590 MPa grade produced by using a steel slab containing V but notcontaining Nb and Ti, V. Moreover, an abscissa in FIGS. 1a and 1 b is anatomic ratio ((V/51)/(C/12)) of V content to C content, and an ordinateis r-value in FIG. 1a and yield ratio (YR) in FIG. 1b.

[0027] As seen from FIGS. 1a and 1 b, a high r-value and a low yieldratio are obtained by limiting V content in the steel slab to a range of0.5-3.0 as the atomic ratio to C content and it is possible to producehigh-strength dual-phase cold rolled steel sheet having an excellentdeep drawability.

[0028] In the steel sheet according to the invention, the inventorsfound that a high r-value is obtained because solid-solute C and N areless and {111} recrystallization texture is strongly developed beforethe recrystallization annealing. And also, the inventors found that byannealing at α-γ two-phase region is dissolved V carbide and thesolid-solute C is enriched into austenite phase in large quantity andthe austenite can be easily transformed into martensite in thesubsequent cooling process to obtain a dual-phase microstructure offerrite and martensite.

[0029] Although Ti and Nb have mainly been used as a carbide formingelement in the past, the inventors paid notice to V having a solubilityof carbide higher than those of Ti and Nb for effectively obtaining thesolid-solute C by annealing at a higher temperature region. That is, itis found that since V carbide easily dissolves as compared with Ticarbide and Nb carbide in the annealing at a high temperature, asufficient amount of solid-solute C for transforming austenite tomartensite is obtained by annealing at the α-γ two-phase region. Inaddition, it is clear that this phenomenon is most remarkably generatedby V, but the similar result is obtained by adding Nb and Ti together.

[0030] Although the invention is based on the above knowledge, thefollowing knowledge is obtained to achieve another invention.

[0031] The inventors compared r-values in the high-strength dual-phasecold rolled steel sheets of TS: 590 MPa grade and TS: 780 MPa producedby using steel slabs containing Nb and Ti in addition to V and madeclear the followings. FIGS. 2a and 2 b show an influence of V, Nb and Ticontents in the steel slab upon tensile strength (TS) and Lankford value(r-value) of a cold rolled steel sheet in the cold rolled steel sheetsof TS: 590 MPa grade and TS: 780 MPa grade produced by using the V, Nband Ti containing steel slab. Moreover, an abscissa in FIGS. 2a and 2 bis an atomic ratio (2×Nb/93+2×Ti/48)/(V/51) of Nb and Ti contents to Vcontent, and an ordinate is tensile strength (TS) in FIG. 2a and r-valuein FIG. 2b.

[0032] According to the above results, in the TS: 780 MPa grade, thehigh-strengthening is attempted by large quantities of solid-solutionstrengthening elements, so that the r-value is lowered as compared withthat of the TS: 590 MPa grade by the increase of the solid-solute Ccontent or the like. In the TS: 780 MPa grade, however, the r-value isconsiderably improved when the value of (2×Nb/93+2×Ti/48)/(V/51) is arange of not less than 1.5. Such a characteristic in the TS: 780 MPagrade that the r-value is remarkably improved when the value of(2×Nb/93+2×Ti/48)/(V/51) is a range of not less than 1.5 is notrecognized in the TS: 590 MPa grade.

[0033] Although the detail of causes on the above result is not clear,it is considered that in the system containing a large amount of anelement resulted in the lowering of the r-value such as solid-solute Cor the like as in the TS: 780 MPa grade, Nb and Ti easily precipitatethe solid-solute C and N as a compound as compared with V and thesolid-solute C and N contents after the hot rolling become less toimprove the r-value. Moreover, when the value of(2×Nb/93+2×Ti/48)/(V/51) exceeds 15, TS considerably lowers, which isunfavorable for obtaining the high-strength dual-phase cold rolled steelsheet of TS: 780 MPa grade. This is considered due to the fact that asNb carbide and Ti are hardly dissolved as compared with V carbide, ifthe addition quantities of the Nb and Ti contents are larger than thatof the V content, the C content enriched in austenite phase is largelydecreased in the annealing at the α-γ two-phase region is widelydecreased and martensite phase generated after the cooling is softened.

[0034] The invention is accomplished by further examining based on theabove knowledge. The summary of the invention is as follows.

[0035] (1) A high-strength dual-phase cold rolled steel sheet having anexcellent deep drawability, characterized in that the steel sheet has acomposition comprising C: 0.01-0.08 mass %, Si: not more than 2.0 mass%, Mn: not more than 3.0 mass %, P: not more than 0.10 mass %, S: notmore than 0.02 mass %, Al: 0.005-0.20 mass %, N: not more than 0.02 mass% and V: 0.01-0.5 mass % provided that V and C satisfy a relationshiprepresented by the following equation (i):

0.5×C/12≦V/51≦3×C/12  (i)

[0036]  and the remainder being Fe and inevitable impurities, and has amicrostructure consisting of a ferrite phase as a primary phase and asecondary phase including martensite phase at an area ratio of not lessthan 1% to a whole of the microstructure.

[0037] (2) A high-strength dual-phase cold rolled steel sheet having anexcellent deep drawability according to the item (1), wherein the steelsheet has a composition comprising further not more than 0.3 mass % intotal of one or tow of Nb: more than 0 mass % but not more than 0.3 mass% and Ti: more than 0 mass % but not more than 0.3 mass % provided thatV, Nb, Ti and C satisfy a relationship represented by the followingequation (ii) instead of the equation (i):

0.5×C/12≦(V/51+2×Nb/93+2×Ti/48)≦3×C/12  (ii)

[0038]  and the remainder being Fe and inevitable impurities.

[0039] Moreover, it is preferable that one or two of Nb: 0.001-3.0 mass% and Ti: 0.001-0.3 mass % is not more than 0.3 mass % in total.

[0040] (3) A high-strength dual-phase cold rolled steel sheet having anexcellent deep drawability according to the item (2), wherein the steelsheet comprises C: 0.03-0.08 mass %, Si: 0.1-2.0 mass %, Mn: 1.0-3.0mass %, P: not more than 0.05 mass % and S: not more than 0.01 mass %and V, Nb and Ti satisfy a relationship of 1.5≦(2×Nb/93+2×Ti/48)/(V/51)≦15.

[0041] (4) A high-strength dual-phase cold rolled steel sheet having anexcellent deep drawability according to any one of the items (1) to (3),wherein the steel sheet further comprises one or two of the following Agroup and B group:

[0042] A group: not more than 2.0 mass % in total of one or two of Crand Mo;

[0043] B group: not more than 2.0 mass % in total of one or two of Cuand Ni.

[0044] (5) A method of producing a high-strength dual-phase cold rolledsteel sheet having an excellent deep drawability, which comprises hotrolling a steel slab having a composition comprising C: 0.01-0.08 mass%, Si: not more than 2.0 mass %, Mn: not more than 3.0 mass %, P: notmore than 0.10 mass %, S: not more than 0.02 mass %, Al: 0.005-0.20 mass%, N: not more than 0.02 mass % and V: 0.01-0.5 mass % provided that Vand C satisfy a relationship represented by the following equation(iii):

0.5×C/12≦V/51≦3×C/12  (iii)

[0045]  and the remainder being Fe and inevitable impurities, pickling,cold rolling and then subjecting to a continuous annealing at atemperature range from a A_(C1) transformation point to a A_(C3)transformation point.

[0046] (6) A method of producing a high-strength dual-phase cold rolledsteel sheet having an excellent deep drawability according to the item(5), wherein the steel sheet has a composition comprising further notmore than 0.3 mass % in total of one or tow of Nb: more than 0 mass %but not more than 0.3 mass % and Ti: more than 0 mass % but not morethan 0.3 mass % provided that V, Nb, Ti and C satisfy a relationshiprepresented by the following equation (iv) instead of the equation(iii):

0.5×C/12≦(V/51+2×Nb/93+2×Ti/48)≦3×C/12  (iv)

[0047]  and the remainder being Fe and inevitable impurities.

[0048] Moreover, it is preferable that one or two of Nb: 0.001-0.3 mass% and Ti: 0.001-0.3 mass % is not more than 0.3 mass % in total.

[0049] (7) A method of producing a high-strength dual-phase cold rolledsteel sheet having an excellent deep drawability according to the item(6), wherein the steel slab comprises C: 0.03-0.08 mass %, Si: 0.1-2.0mass %, Mn: 1.0-3.0 mass %, P: not more than 0.05 mass % and S: not morethan 0.01 mass % and V, Nb and Ti satisfy a relationship of1.5≦(2×Nb/93+2×Ti/48)/(V/51)≦15.

[0050] (8) A method of producing a high-strength dual-phase cold rolledsteel sheet having an excellent deep drawability according to any one ofthe items (5)-(7), wherein the steel slab further comprises one or twoof the following A-group and B-group:

[0051] A-group: not more than 2.0 mass % in total of one or two of Crand Mo;

[0052] B-group: not more than 2.0 mass % in total of one or two of Cuand Ni.

[0053] (9) A high-strength dual-phase galvanized steel sheet having anexcellent deep drawability comprising a galvanized coating on the steelsheet disclosed in any one of the items (1)-(4).

[0054] (10) A method of producing a high-strength dual-phase galvanizedsteel sheet having an excellent deep drawability, wherein agalvanization is carried out after the continuous annealing at atemperature range from a A_(C1) transformation point to a A_(C3)transformation point in the production method described in any one ofthe items (5)-(7).

[0055] (11) A method of producing a high-strength dual-phase galvanizedsteel sheet having an excellent deep drawability according to the item(10), which further comprising a continuous annealing step between thecold rolling step and the continuous annealing step at a temperaturerange from a A_(C1) transformation point to a A_(C3) transformationpoint.

[0056] (12) A method of producing a high-strength dual-phase galvanizedsteel sheet having an excellent deep drawability according to the item(10) or (11), wherein the steel slab further comprises one or two of thefollowing A-group and B-group:

[0057] A-group: not more than 2.0 mass % in total of one or two of Crand Mo;

[0058] B-group: not more than 2.0 mass % in total of one or two of Cuand Ni.

[0059] The cold rolled steel sheet and the galvanized steel sheetaccording to the invention are high-strength dual-phase steel sheetshaving a tensile strength (TS) of not less than 440 MPa and an excellentdeep drawability.

[0060] At first, the reason of limiting the composition in the coldrolled steel sheet and the galvanized steel sheet according to theinvention will be explained below. Moreover, mass % represents simply as“%”.

[0061] C: 0.01-0.08%

[0062] C is an element for increasing the strength of the steel sheetand further promoting the formation of a dual-phase microstructure offerrite and martensite, and is necessary to contain not less than 0.01%,preferably not less than 0.015% from a viewpoint of the formation of thedual-phase microstructure in the invention. Moreover, if it is intendedto increase the strength to TS: not less than 540 MPa and TS: not lessthan 780 MPa, the C content is preferable to be not less than 0.015% andnot less than 0.03%, respectively. On the other hand, when the C contentexceeds 0.08%, the development of {111} recrystallization texture isobstructed to degrade the deep drawability. Therefore, the inventionlimits the C content to 0.01-0.08%. When it is particularly required toincrease the strength of the steel sheet, it is preferable to be0.03-0.08%. Moreover, it is preferable to be not more than 0.05% from aviewpoint of the deep drawability.

[0063] Si: Not More Than 2.0%

[0064] Although Si is a useful reinforcing element capable of increasingthe strength of the steel sheet without remarkably lowering theductility of the steel sheet, if the content exceeds 2.0%, thedeterioration of the deep drawability is caused, but also the surfaceproperties are degraded. Therefore, Si is limited to not more than 2.0%.Moreover, if it is intended to increase the strength to TS: not lessthan 780 MPa, it is preferable to be not less than 0.1% for ensuring therequired strength. And also, it is preferable to be not less than 0.01%for increasing the strength to TS: not less than 440 MPa which is a mainobject of the invention.

[0065] Mn: Not More Than 3.0%

[0066] Mn has an action reinforcing the steel and further has an actionof lessening a critical cooling rate for the obtention of the dual-phasemicrostructure of ferrite and martensite to promote the formation of thedual-phase microstructure of ferrite and martensite, so that it ispreferable to contain a content in accordance with the cooling rateafter the recrystallization annealing. And also, Mn is an effectiveelement preventing the hot tearing through S, so that it is preferableto contain an appropriate content in accordance with S content. However,when the Mn content exceeds 3.0%, the deep drawability and weldabilityare degraded. In the invention, therefore, the Mn content is limited tonot more than 3.0%. Moreover, the Mn content is preferable to be notless than 0.5% for remarkably developing the above effect, andparticularly it is preferable to be not less than 1.0% for increasingthe strength to TS: not less than 780 MPa. And also, it is preferable tobe not less than 0.1% for increasing the strength to TS: not less than440 MPa which is a main object of the invention.

[0067] P: Not More Than 0.10%

[0068] P has an action reinforcing the steel and can be contained in arequired amount in accordance with the desired strength. When the Pcontent exceeds 0.10%, the press formability is degraded. Therefore, theP content is limited to not more than 0.10%. Moreover, if a moreexcellent press formability is required, the P content is preferable tobe not more than 0.08%. Furthermore, when large quantities of C, Mn andthe like are contained in order to ensure TS: not less than 780 MPa, theP content is preferable to be not more than 0.05% in order to preventthe degradation of the weldability. In addition, if it is intended toincrease the strength to TS: not less than 440 MPa, it is preferable tobe not less than 0.001%.

[0069] S: Not More Than 0.02%

[0070] S is existent as an inclusion in the steel sheet and is anelement bringing about the degradation of the ductility and theformability of the steel sheet, particularly the stretch-flangingproperty. Therefore, it is preferable to be decreased as far aspossible, and when it is decreased to not more than 0.02%, S does notexert a bad influence, so that the S content is 0.02% as an upper limitin the invention. Moreover, when the more excellent stretch-flangingproperty is required, or when the large quantities of C, Mn and the likeare contained in order to ensure TS: not less than 780 MPa, if theexcellent weldability is required, the S content is preferable to be notmore than 0.01%, more preferably not more than 0.005%. On the otherhand, the S content is preferable to be not less than 0.0001%considering a cost for the removal of S in the steelmaking process.

[0071] Al: 0.005-0.20%

[0072] Al is added to the steel as a deoxidizing element and is a usefulelement for improving the cleanliness of the steel, but the additioneffect is not obtained at less than 0.005%. On the other hand, when itexceeds 0.20%, the more deoxidizing effect is not obtained and the deepdrawability is inversely degraded. Therefore, the Al content is limitedto 0.005-0.20%. Moreover, the invention does not exclude a steelmakingmethod through deoxidization other than the Al deoxidization. Forexample, Ti deoxidization or Si deoxidization may be conducted. Thesteel sheets made by these deoxidizing methods are included within ascope of the invention. In this case, even if Ca, REM and the like areadded to the molten steel, the characteristics of the steel sheetaccording to the invention are not obstructed, so that the steel sheetincluding Ca, REM and the like is naturally included within the scope ofthe invention.

[0073] N: Not More Than 0.02%

[0074] N is an element increasing the strength of the steel sheet by thesolid-solution hardening and the strain ageing hardening, but when Ncontent exceeds 0.02%, the nitride is increased in the steel sheet toremarkably degrade the deep drawability of the steel sheet. Therefore,the N content is limited to not more than 0.02%. Moreover, in case ofrequiring the more improvement of the press formability, the N contentis preferable to be not more than 0.01%, more preferably not more than0.004%. In this case, considering the cost for denitrification in thesteelmaking process, the N content is preferable to be not less than0.0001%.

[0075] V: 0.01-0.5% and 0.5×C/12≦V/51≦3×C/12

[0076] V is a most important element in the invention. Before therecrystallization, the solid-solute C is precipitated and fixed as Vcarbide to develop the {111} recrystallization texture, whereby a highr-value can be obtained. Moreover, V dissolves the V carbide in theannealing at α-γ two-phase region to enrich a large quantity of thesolid-solute C in austenite phase, which is easily transformed intomartensite at the subsequent cooling process, whereby the dual-phasesteel sheet having a dual-phase microstructure of ferrite and martensitecan be obtained. Such an effect becomes effective when the V content isnot less than 0.01%, more preferably not less than 0.02% and satisfies0.5×C/12≦V/51 in relation to the C content. On the other hand, when theV content exceeds 0.5% or when it is V/51>3×C/12 in relation to the Ccontent, the dissolution of the V carbide at the α-γ two-phase regionhardly occurs and the dual-phase microstructure of ferrite andmartensite is hardly obtained. Therefore, the V content is limited to0.01-0.5% and to 0.5×C/12≦V/51≦3×C/12. Moreover, V/51≦2×C/12 ispreferable for obtaining the dual-phase microstructure of ferrite andmartensite.

[0077] In addition to the above composition, it is further preferable tocontain not more than 0.3 (mass) % in total of one or two of Nb: morethan 0% but not more than 0.3 (mass) % and Ti: more than 0% but not morethan 0.3%, and that V, Nb, Ti contents satisfy0.5×C/12≦(V/51+2×Nb/93+2×Ti/48)≦3×C/12 in relation to the C content inplace of that the V and C content satisfy 0.5×C/12≦V/51≦3×C/12.

[0078] Not More Than 0.3% in Total of One or Tow of Nb: More Than 0% butnot More Than 0.3% and Ti: More Than 0% but not More Than 0.3%, and V,Nb, Ti and C Satisfy 0.5×C/12≦(V/51+2−Nb/93+2×Ti/48)≦3×C/12

[0079] Nb and Ti are carbide forming elements likewise V and have thesame action as V mentioned above. That is, a high r-value can beobtained by precipitating and fixing the solid-solute C as Nb and Ticarbides before the recrystallization to develop the {111}recrystallization texture, and also a dual-phase steel sheet having adual-phase microstructure of ferrite and martensite can be obtained bydissolving the Nb and Ti carbides in the annealing at the α-γ two-phaseregion to enrich a large quantity of the solid-solute C in austenitephase and transforming into martensite in the subsequent coolingprocess. Moreover, as the above effect of Nb and Ti is considerablysmall as compared with that of V, when only Nb and Ti are added to thesteel slab without adding V, the deep drawability aiming at theinvention can not be enhanced sufficiently.

[0080] Therefore, it is preferable to add Nb and Ti of more than 0%.More preferably, each of the Nb and Ti contents is not less than 0.001%.In this case, it is preferable to satisfy0.5×C/12≦(V/51+2×Nb/93+2×Ti/48) in relation to the C and V contents fordeveloping the above effect. On the other hand, when each of Nb and Ticontents or both in total thereof exceeds 0.3%, or when the Nb and Ticontents satisfy (V/51+2×Nb/93+2×Ti/48)>3×C/12 in relation to the C andV contents, the dissolution of the carbide at the α-γ two-phase regionhardly occurs and hence the dual-phase microstructure of ferrite andmartensite is hardly obtained. Therefore, it is preferable that wheneither Nb or Ti is merely added, each of the Nb content and the Ticontent is within a range of more than 0% but not more than 0.3%, andwhen both of Nb and Ti are added together, the Nb and Ti contents arenot more than 0.3% in total and satisfy0.5×C/12≦(V/51+2×Nb/93+2×Ti/48)≦3×C/12 in relation to the V and Ccontents.

[0081] On the other hand, if it is intended to increase the strength toTS: not less than 780 MPa, the deep drawability is apt to be easilydegraded by the addition of large quantities of solid-solutionstrengthening elements such as C, Mn and the like. In this case, the V,Nb and Ti contents are further desirable to be a range of1.5≦(2×Nb/93+2×Ti/48)/(V/51)≦15. The reason why (2×Nb/93+2×Ti/48)/(V/51)is limited to not less than 1.5 is considered due to the fact thatalthough the detail of the cause is not clear, the formation of carbideafter the hot rolling is promoted to decrease the solid-solute C byadding large quantities of Nb and Ti as compared with V and hence the{111} recrystallization texture is easily developed. Moreover, in orderto ensure the strength of TS: not less than 780 MPa,(2×Nb/93+2×Ti/48)/(V/51) is desirable to be a range of not more than 15.

[0082] Furthermore, in addition to the above steel composition, thesteel according to the invention is preferable to further comprise oneor two of the following A-group and B-group:

[0083] A-group: not more than 2.0% in total of one or two of Cr and Mo;

[0084] B-group: not more than 2.0% in total of one or two of Cu and Ni.

[0085] A-Group: Not More Than 2.0% in Total of One or Two of Cr and Mo

[0086] All of Cr and Mo in the A-group have an action of decreasing thecritical cooling rate for providing the dual-phase microstructure offerrite and martensite to promote the formation of the dual-phasemicrostructure of ferrite and martensite likewise Mn and can beincluded, if necessary. The lower limits of the Cr content and Mocontent preferable for obtaining the above effect are Cr: 0.05%, Mn:0.05%. However, when one or two of Cr and Mo exceed 2.0% in total, thedeep drawability is degraded. To this end, one or more of Cr and Mo inthe A-group is preferable to be limited to not more than 2.0% in total.

[0087] B-Group: Not More Than 2.0% in Total of One or Two of Cu and Ni

[0088] Cu and Ni in the B-group have an action of reinforcing the steeland may be included at a required amount in accordance with the desiredstrength. However, when the content of Cu and Ni added alone or togetherexceeds 2.0% in total, it tends to degrade the deep drawability. To thisend, one or more of Cu and Ni is preferable to be not more than 2.0% intotal. Moreover, the lower limits of the Cu and Ni contents preferablefor obtaining the above effect is Cu: 0.05% and Ni: 0.05%, respectively.

[0089] Although elements other than the above elements are notparticularly limited in the invention, there is no problem even if B,Ca, Zr, REM and the like is included within a range of the usual steelcomposition.

[0090] In this case, B is an element having an action of improving thehardenability in the steel and may be included, if necessary. However,when the B content exceeds 0.003%, the above effect is saturated, sothat the B content is preferable to be not more than 0.003%. Moreover, amore desirable range is 0.001-0.002%. Ca and REM have an action ofcontrolling the form of sulfide inclusion and also have an effect ofimproving the stretch-flanging property. Such an effect is saturatedwhen one or two selected from Ca and REM exceed 0.01% in total. To thisend, the content of one or two of Ca and REM is preferable to be notmore than 0.01% in total. Moreover, a more preferable range is0.001-0.005%.

[0091] The reminder other than the above elements is Fe and inevitableimpurities. As the inevitable impurity are mentioned, for example, Sb,Sn, Zn, Co and the like. As acceptable ranges of their contents are Sb:not more than 0.01%, Sn: not more than 0.1%, Zn: not more than 0.01% andCo: not more than 0.1%.

[0092] Next, the microstructure of the steel sheet according to theinvention will be explained.

[0093] The cold rolled steel sheet according to the invention has amicrostructure consisting of ferrite phase as a primary phase and asecondary phase including not less than 1% of martensite phase at anarea ratio with respect to a whole of the microstructure.

[0094] In order to provide the cold rolled steel sheet having a lowyield stress (YS), a high ductility (E1) and an excellent deepdrawability, it is required to render the microstructure of the steelsheet according to the invention into a dual-phase microstructureconsisting of a ferrite phase as a primary phase and a secondary phaseincluding a martensite phase. It is preferable that the ferrite phase asa primary phase is not less than 80% at an area ratio and hence thesecondary phase is not more than 20%. When the area ratio of the ferritephase is less than 80%, it is difficult to ensure the high ductility andthe press formability tends to lower. And also, when a good ductility isrequired, it is preferable that the ferrite phase is not less than 85%at the area ratio and hence the secondary phase is not more than 15%.Moreover, in order to utilize the advantage of the dual-phasemicrostructure, the ferrite phase is required to be not more than 99%.

[0095] In the invention, the secondary phase is required to include themartensite phase at the area ratio of not less than 1% with respect tothe whole of the microstructure. When the martensite is less than 1% atthe area ratio, the low yield stress (YS) and the high ductility (E1)can not be satisfied simultaneously. More preferably, the martensitephase is not less than 3% but not more than 20% at the area ratio. Incase of requiring a good ductility, the martensite phase is preferableto be not more than 15% at the area ratio. Moreover, the secondary phasemay be constituted by only the martensite phase at the area ratio of notless than 1% or by mixed phases of the martensite phase at the arearatio of not less than 1% and any of a pearlite phase, a bainite phaseand a retained austenite as an additional phase and is not especiallylimited. In the latter case, the pearlite phase, the bainite phase andthe retained austenite are preferable to be not more than 50% in totalat the area ratio with respect to the microstructure of the secondaryphase in order to more effectively develop the effect of the martensitephase.

[0096] The cold rolled steel sheet and the galvanized steel sheet havingthe above microstructure are steel sheets having a low yield stress, ahigh ductility and an excellent deep drawability.

[0097] Next, the method of producing the cold rolled steel sheet and thegalvanized steel sheet according to the invention will be explained.

[0098] The composition of the steel slab used in the production methodof the invention is the same as the compositions of the aforementionedcold rolled steel sheet and the galvanized steel sheet, so that theexplanation on the reason of the limitation in the steel slab isomitted.

[0099] The cold rolled steel sheet according to the invention isproduced by using a steel slab having a composition of the above rangeas a starting material and successively subjecting this startingmaterial to a hot rolling step of subjecting to a hot rolling to obtaina hot rolled steel sheet, a pickling step of pickling the hot rolledsteel sheet, a cold rolling step of subjecting the hot rolled steelsheet to a cold rolling to obtain a cold rolled steel sheet, and arecrystallization annealing step of subjecting the cold rolled steelsheet to a recrystallization annealing to obtain a cold rolled annealedsteel sheet.

[0100] And also, the galvanized steel sheet according to the inventionis produced by using a steel slab having a composition of the aboverange as a starting material and successively subjecting this startingmaterial to a hot rolling step of subjecting to a hot rolling to obtaina hot rolled steel sheet, a pickling step of pickling the hot rolledsteel sheet, a cold rolling step of subjecting the hot rolled steelsheet to a cold rolling to obtain a cold rolled steel sheet, and acontinuous galvanization step of subjecting the cold rolled steel sheetto a recrystallization annealing and a galvanizing to obtain agalvanized steel sheet. Furthermore, it is produced by subjecting thecold rolled steel sheet to a step of annealing and pickling before thecontinuous galvanization step, if necessary.

[0101] The steel slab used is preferable to be produced by a continuouscasting process in order to prevent the macro-segregation of thecomponents, but may be produced by an ingot casting process or a thinslab casting process. Furthermore, in addition to the conventionalprocess of cooling to a room temperature once after the production ofthe steel slab and again heating, energy-saving processes such as aprocess for inserting a hot steel slab into a heating furnace withoutcooling, a process for direct sending rolling or direct rollingimmediately after slight heat-holding and the like can be appliedwithout problems.

[0102] The above starting material (steel slab) is subjected to the hotrolling step of forming the hot rolled steel sheet by heating and hotrolling. In the hot rolling step, there is particularly no problem evenin the use of usual rolling conditions as long as the hot rolled steelsheet having a desired thickness can be produced. Moreover, preferablehot rolling conditions are mentioned below for the reference.

[0103] Slab Heating Temperature: not Lower Than 900° C.

[0104] The slab heating temperature is desirable to be made lower as faras possible in order to improve the deep drawability by coarsening theprecipitate to develop the {111} recrystallization texture. However,when the slab heating temperature is lower than 900° C., the rollingload increases and the risk of causing troubles in the hot rollingincreases. To this end, the slab heating temperature is preferable to benot lower than 900° C. And also, the upper limit of the slab heatingtemperature is more preferable to be 1300° C. in terms of the loweringof the yield resulted from the increase of scale loss accompanied withthe increase of the oxide weight. Moreover, it goes without saying thatthe utilization of a so-called sheet bar heater of heating the sheet barin the hot rolling is an effective process from a viewpoint that theslab heating temperature is lowered and the troubles in the hot rollingare prevented.

[0105] Finisher Delivery Temperature: not Lower Than 700° C.

[0106] The finisher delivery temperature (FDT) is preferable to be notlower than 700° C. in order to obtain a uniform microstructure of thehot rolled parent sheet for providing an excellent deep drawabilityafter the cold rolling and the recrystallization annealing. That is,when the finish deformation temperature is lower than 700° C., not onlythe microstructure of the hot rolled parent sheet becomes nonuniform,but also the rolling load in the hot rolling becomes higher and the riskof causing the trouble in the hot rolling is increased.

[0107] Coiling Temperature: not More Than 800° C.

[0108] The coiling temperature is preferable to be not higher than 800°C. That is, when the coiling temperature exceeds 800° C., the scaleincreases and the yield tends to lower due to the scale loss. And also,when the coiling temperature is lower than 200° C., the shape of thesteel sheet remarkably is disordered and the risk of causing problems inthe actual use increases, so that the lower limit of the coilingtemperature is more preferable to be 200° C.

[0109] As mentioned above, in the hot rolling step according to theinvention, it is preferable that the steel slab is heated above 900° C.,subjected to the hot rolling at the finish deformation temperature ofnot lower than 700° C., and coiled at the coiling temperature of nothigher than 800° C.

[0110] Moreover, in the hot rolling step according to the invention, alubrication rolling may be conducted in a part of the finish rolling orbetween passes thereof in order to reduce the rolling load in the hotrolling. In addition, the application of the lubrication rolling iseffective from a viewpoint of the uniformization of the steel sheetshape and the homogenization of the material. Also, the coefficient offriction in the lubrication rolling is preferable to be within a rangeof 0.10-0.25.

[0111] Further, the hot rolling step is preferable to be a continuousrolling process wherein the sheet bars located in front and rear arejoined to each other and continuously subjected to the finish rolling.The application of the continuous rolling process is desirable from aviewpoint of the operating stability in the hot rolling.

[0112] Next, the hot rolled steel sheet is subjected to the pickling forthe removal of the scale. The pickling step is sufficient according tothe usual manner and it is preferable to use a treating solution such ashydrochloric acid, sulfuric acid or the like as a pickling solution.

[0113] Moreover, the cold rolled steel sheet is formed by subjecting thehot rolled steel sheet to the cold rolling. The cold rolling conditionsare not especially limited as long as the cold rolled steel sheet havingdesired size and shape can be obtained, but it is preferable that arolling reduction in the cold rolling is not less than 40%. When therolling reduction is less than 40%, the {111} recrystallization textureis not developed and the excellent deep drawability can not be obtained.

[0114] The cold rolled steel sheet according to the invention issubjected to a recrystallization annealing in the subsequentrecrystallization annealing step to obtain a cold rolled annealed steelsheet. The recrystallization annealing is carried out in a continuousannealing line. On the other hand, the galvanized steel sheet accordingto the invention is produced by subjecting the cold rolled steel sheetto recrystallization annealing and galvanizing in the continuousgalvanization line after the cold rolling. In this case, the annealingtemperature in the recrystallization annealing is required to beconducted at a (α+γ) two-phase region within a temperature range fromA_(C1) transformation point to A_(C3) transformation point. This is dueto the fact that the annealing is carried out at (α+γ) two-phase regionto dissolve the carbides of V, Ti and Nb to thereby distribute an amountof solid-solute C sufficient to transform austenite to martensite intothe austenite phase. When the annealing temperature is lower than theA_(C1) transformation point, the microstructure is rendered into theferrite single phase and the martensite can not be generated, while whenit is higher than the A_(C3) transformation point, the crystal grainsare coarsened and the microstructure is rendered into the austenitesingle phase and the {111} recrystallization texture is not developedand hence the deep drawability is deteriorated remarkably.

[0115] In the cold rolled steel sheet according to the invention, thecooling in the recrystallization annealing is preferable to be conductedat a cooling rate of not less than 5° C./s in order to produce themartensite phase to obtain the dual-phase microstructure of ferrite andmartensite.

[0116] On the other hand, in the galvanized steel sheet according to theinvention, it is preferable to quench to a temperature region of380-530° C. after the above recrystallization annealing. When a stoptemperature of the quenching is lower than 380° C., the defectiveplating easily occurs, while when it exceeds 530° C., the unevennesseasily occurs on the plated surface. Moreover, the cooling rate ispreferable to be not less than 5° C./s in order to produce themartensite phase to obtain the dual-phase microstructure of ferrite andmartensite. After the above quenching, the galvanization is carried outby dipping in a galvanizing bath. In this case, Al concentration in thegalvanizing bath is preferable to be within a range of 0.12-0.145 mass%. When the Al concentration in the galvanizing bath is less than 0.12mass %, the alloying excessively advances and the plating adhesion(resistance to powdering) tends to be deteriorated, while when itexceeds 0.145 mass %, the defective plating easily occurs.

[0117] And also, the plated layer may be subjected to an alloyingtreatment after the galvanization. Moreover, the alloying treatment ispreferable to be conducted so that Fe content in the plated layer is9-12%.

[0118] As the alloying treatment, it is preferable to conduct thealloying of the galvanized layer by reheating up to a temperature regionof 450-550° C. After the alloying treatment, it is preferable to cool ata cooling rate of not less than 5° C./s to 300° C. The alloying at ahigh temperature is difficult to form the martensite phase and there iscaused a fear of degrading the ductility of the steel sheet, while whenthe alloying temperature is lower than 450° C., the progress of thealloying is slow and the productivity tends to lower. Furthermore, whenthe cooling rate after the alloying treatment is extremely small, theformation of the martensite becomes difficult. To this end, the coolingrate at a temperature region from after the alloying treatment to 300°C. is preferable to be not less than 5° C./s.

[0119] Moreover, if it is required to further improve the platingproperty, it is preferable that after the cold rolling and before beingsubjected to the continuous galvanization, the annealing is separatelyconducted in the continuous annealing line and subsequently an enrichedlayer of components in the steel produced on the surface of the steelsheet is removed by pickling and thereafter the above treatment isconducted in the continuous galvanization line. In this case, thepickling may be carried out in the pickling line or in the pickling batharranged in the continuous galvanization line. Also, the atmosphere inthe continuous annealing line is preferable to be a reducing atmospherewith respect to the steel sheet in order to prevent the formation of thescale, and it is generally sufficient to use a nitrogen gas containingseveral % of H₂. The annealing is preferable to be conducted under acondition that a temperature of the steel sheet reaching in thecontinuous annealing line is not lower than the A_(C1) transformationpoint decided by the components in the steel. Because it is required topromote the enrichment of the alloying element on the surface of thesteel sheet and to enrich the alloying element in the secondary phase byonce forming the dual-phase microstructure in the continuous annealingline. In the steel sheet after the annealing in the continuous annealingline, there is a tendency that P among the components in the steel isdiffused to segregate on the surface of the steel sheet and Si, Mn, Crand the like enrich as an oxide, so that it is preferable to remove theenriched layer formed on the surface of the steel sheet by the pickling.Then, the same annealing as in the above is performed in the continuousgalvanization line. In order to develop the characteristics as thedual-phase microstructure, the annealing in the continuous galvanizationline is preferable to be performed at (α+γ) two-phase region within atemperature range of from the A_(C1) transformation point to the A_(C3)transformation point. In this case, the reason why the annealing isperformed at not lower than the A_(C1) transformation point in both thecontinuous annealing line and the continuous galvanization line is dueto the fact that the dual-phase microstructure is formed as mentionedabove. Once an enriching place of the element as the secondary phase isformed by forming the dual-phase microstructure as a finalmicrostructure in the continuous annealing line, it becomes possible toenrich the alloying element to some degree at this place. Desirably, itis sufficient to obtain the same dual-phase microstructure as in a finalproduct after the cooling, so that the alloying element is morepreferable to be enriched in the vicinity of a triple point of grainboundary (intersection of the grain boundary formed by three crystalgrains). Thereafter, when the annealing is performed at the two-phaseregion in the continuous galvanization line, the alloying element isfurther enriched in the secondary phase or γ-phase and hence the γ-phaseeasily transforms into the martensite phase during the cooling process.Moreover, the term “alloying element” used herein means a substitutionalalloying element such as Mn, Mo or the like, which makes a situationthat diffusion hardly occurs and enrichment easily occurs at thetemperature in the annealing step in order to lower the yield ratio.

[0120] And also, the cold rolled steel sheet after the recrystallizationannealing process and the galvanized steel sheet after the platingprocess or after the alloying process may be subjected to a temperrolling at a rolling reduction of not more than 10% for correcting theshape and adjusting the surface roughness and the like. Furthermore, thecold rolled steel sheet according to the invention can be applied as notonly a cold rolled steel sheet for the working but also a blank of asurface treated steel sheet for the working. As the surface treatedsteel sheet for the working are mentioned tin-plated steel sheets,porcelain enamels and so on in addition to the aforementioned galvanizedsteel sheets (including alloyed sheets). There is no problem even whenthey are subjected to a treatment such as resin or fat coating, variouspaintings, electroplating or the like. Moreover, the galvanized steelsheet according to the invention may be subjected to a special treatmentafter the galvanization in order to improve the chemical conversionproperty, weldability, press formability, corrosion resistance and thelike.

BRIEF DESCRIPTION OF THE DRAWINGS

[0121]FIG. 1a is a graph showing an influence of V and C contents insteel upon a Lankford value (r-value).

[0122]FIG. 1b is a graph showing an influence of V and C contents insteel upon a yield ratio (YR=yield stress(YS)/tensilestress(TS)×100(%)).

[0123]FIG. 2a is a graph showing an influence of a relationship amongNb, Ti and V contents upon a tensile strength (TS) in the high-strengthdual-phase cold rolled steel sheets of TS: 590 MPa grade and TS: 780 MPagrade.

[0124]FIG. 2b is a graph showing an influence of a relationship amongNb, Ti and V contents upon a Lankford value (r-value) in thehigh-strength dual-phase cold rolled steel sheets of TS: 590 MPa gradeand TS: 780 MPa grade.

BEST MODE FOR CARRYING OUT THE INVENTION

[0125] Each of molten steels having compositions shown in Tables 1-4 ismade in a converter and subjected to a continuous casting process toobtain a slab. In this case, each of the slabs having the compositionsshown in Tables 1 and 2 is prepared for the purpose of experiments withrespect to the cold rolled steel sheet, and each of the slabs having thecompositions shown in Tables 3 and 4 is prepared for the purpose ofexperiments with respect to the galvanized steel sheet. Especially, theslabs shown in Tables 2 and 4 are prepared for the purpose of obtainingthe cold rolled steel sheet and galvanized steel sheet of TS: not lessthan 780 MPa, respectively. Then, the steel slab is heated to 1150° C.and subjected to a hot rolling under conditions of a finish deformationtemperature: 900° C. and a coiling temperature: 650° C. at a hot rollingstep to obtain a hot rolled steel strip having a thickness of 4.0 mm.Subsequently, the hot rolled steel strip is pickled and subjected to acold rolling at a rolling reduction of 70% at a cold rolling step toobtain a cold rolled steel strip or a cold rolled sheet having athickness of 1.2 mm. Next, each of the cold rolled steel sheets inTables 1 and 2 is subjected to a recrystallization annealing at anannealing temperature shown in Tables 5 and 6 in a continuous annealingline. The thus obtained cold rolled sheet is further subjected to atemper rolling at a rolling reduction of 0.8%. With respect to thegalvanized steel sheets, each of the cold rolled sheets in Tables 3 and4 is subjected to a recrystallization annealing at an annealingtemperature shown in Tables 7 and 8 and further to a galvanizing in agalvanizing bath having an Al concentration of 0.13% in a continuousgalvanization line. Moreover, with respect to a part of steel sheets(Steel sheet Nos. 52, 68, 69 and 70 in Table 7), the steel sheet afterthe cold rolling is subjected to an annealing at 830° C. in a continuousannealing line and then pickled and annealed and galvanized at agalvanizing bath temperature of 480° C. under an Al concentration in thebath of 0.13% in a continuous galvanization line and further the thusobtained steel strip (galvanized steel sheet) is subjected to a temperrolling at a rolling reduction of 0.8%. With respect to the steel sheets75 and 77 in Table 7, they are subjected to an alloying treatment at analloying temperature of 520° C. after the galvanization.

[0126] A test piece is cut out from the obtained steel strip and amicrostructure thereof with respect to a section (C section)perpendicular to the rolling direction is imaged by using an opticalmicroscope or a scanning electron microscope to measure a structureratio of ferrite phase as a primary phase and a kind and a structureratio of a secondary phase by using an image analysis device. In thiscase, a specimen for observing the microstructure is subjected to amirror-like polishing and an etching with an alcohol solution containing2% HNO₃ and then used for the observation. And also, a tensile testpiece of JIS No. 5 is cut out from the steel strip and subjected to atensile test according to the definition of JIS Z 2241 to measure ayield stress (YS), a tensile strength (TS), an elongation (E1), a yieldratio (YR) and a Lankford value (r-value). These results are shown inTables 5-8. TABLE 1(a) Steel Chemical composition (mass %) No. C Si Mn PS Al N V Nb Ti Cr Mo Cu Ni 1-A 0.030 0.02 1.55 0.01 0.004 0.032 0.0020.132 — — — — — — 1-B 0.028 0.02 1.48 0.01 0.001 0.032 0.002 0.105 0.042— — 0.15 — — 1-C 0.032 0.03 1.72 0.01 0.005 0.028 0.002 0.085 0.0350.035 0.05 — — — 1-D 0.020 0.02 1.63 0.01 0.005 0.033 0.002 0.065 — — —— 0.12 0.08 1-E 0.031 0.02 1.56 0.01 0.006 0.033 0.002 0.122 0.045 — —0.18 — — 1-F 0.029 0.02 1.48 0.01 0.003 0.032 0.002 0.210 0.115 0.125 —— — — 1-G 0.032 0.02 1.65 0.01 0.004 0.032 0.002 0.045 — — — — — — 1-H0.020 0.22 2.02 0.06 0.004 0.032 0.002 0.132 — — — — — 1-I 0.022 0.521.85 0.03 0.001 0.032 0.002 0.105 0.042 — — 0.15 — — 1-J 0.028 0.33 1.720.01 0.005 0.028 0.002 0.085 0.035 0.035 0.05 — — — 1-K 0.011 0.21 1.530.01 0.003 0.028 0.002 0.032 0.030 — — — — — 1-L 0.022 0.52 1.52 0.010.002 0.033 0.002 0.125 — 0.022 — — — — 1-M 0.019 0.53 1.43 0.05 0.0010.032 0.002 0.105 — — 0.05 0.15 — — Transformation point Steel (° C.)No. X*¹ Y*² Z*³ A_(c1) A_(c3) Remarks 1-A 1.04 — — 725 860 Acceptableexample 1-B — 1.27 0.44 705 855 Acceptable example 1-C — 1.45 1.33 710850 Acceptable example 1-D 0.76 — — 715 855 Acceptable example 1-E —1.30 0.40 705 855 Acceptable example 1-F — 4.88 1.26 725 855 Comparativeexample 1-G 0.33 — — 715 850 Comparative example 1-H 1.55 — — 725 860Acceptable example 1-I — 1.62 0.44 705 865 Acceptable example 1-J — 1.661.33 710 860 Acceptable example 1-K — 1.39 1.03 710 860 Acceptableexample 1-L — 1.84 0.37 715 865 Acceptable example 1-M 1.30 — — 710 850Acceptable example

[0127] TABLE 1(b) Steel Chemical composition (mass %) No. C Si Mn P S AlN V Nb Ti Cr Mo Cu Ni 1-N 0.021 0.33 1.72 0.06 0.003 0.030 0.002 0.115 —— 0.05 0.15 0.15 0.15 1-O 0.020 0.41 2.02 0.02 0.002 0.029 0.002 0.0720.042 0.010 0.05 0.15 0.10 0.10 1-P 0.007 0.35 1.76 0.01 0.005 0.0290.002 0.073 — — — — — — 1-Q 0.112 0.33 1.74 0.01 0.003 0.028 0.002 0.352— — — — — — 1-R 0.021 0.52 1.52 0.01 0.002 0.033 0.002 0.008 — — — — — —1-S 0.023 0.53 1.43 0.05 0.001 0.032 0.002 0.622 — — — — — — 1-T 0.0210.33 1.72 0.06 0.003 0.030 0.002 0.049  0.0005 — — — — — 1-U 0.025 0.411.75 0.04 0.002 0.029 0.002 0.041 0.325 — — — — — 1-V 0.019 0.35 1.760.05 0.001 0.032 0.002 0.052 —  0.0005 — — — — 1-W 0.023 0.33 1.72 0.060.003 0.030 0.002 0.033 — 0.306 — — — — 1-X 0.018 0.02 1.48 0.01 0.0030.032 0.002 0.030 0.001 0.001 — — — — 1-Y 0.021 0.02 1.65 0.01 0.0040.032 0.002 0.329 — — — — — — Transformation point Steel (° C.) No. X*¹Y*² Z*³ A_(c1) A_(c3) Remarks 1-N 1.29 — — 705 855 Acceptable example1-O — 1.64 0.93 715 850 Acceptable example 1-P 2.45 — — 714 882Comparative example 1-Q 0.74 — — 714 859 Comparative example 1-R 0.09 —— 722 879 Comparative example 1-S 6.36 — — 723 972 Comparative example1-T — 0.56 0.01 714 910 Acceptable example 1-U — 3.74 8.69 716 891Comparative example 1-V — 0.66 0.02 714 906 Acceptable example 1-W —6.99 19.7 714 1033 Comparative example 1-X — 0.43 0.07 708 863Comparative example 1-Y 3.69 — — 706 886 Comparative example

[0128] TABLE 2 Steel Chemical composition (mass %) No. C Si Mn P S Al NV Nb Ti Cr Mo Cu Ni 2-A 0.039 0.50 2.85 0.01 0.005 0.031 0.002 0.151 — —— — — — 2-B 0.038 0.75 2.52 0.01 0.001 0.035 0.002 0.088 0.121 — — 0.29— — 2-C 0.042 0.74 2.53 0.01 0.006 0.033 0.002 0.092 0.110 0.152 0.09 —— — 2-D 0.041 0.70 2.55 0.01 0.008 0.032 0.002 0.087 — 0.064 — — 0.080.1 2-E 0.048 0.72 2.52 0.01 0.005 0.034 0.002 0.153 0.202 0.005 — 0.31— — 2-F 0.040 0.77 2.55 0.01 0.007 0.036 0.002 0.524 0.193 0.262 — — — —2-G 0.038 0.73 2.56 0.01 0.006 0.033 0.002 0.040 0.011 0.009 — — — — 2-H0.043 0.95 2.95 0.05 0.005 0.032 0.002 0.095 0.002 0.119 — 0.27 — — 2-I0.042 0.82 2.78 0.04 0.009 0.035 0.002 0.141 0.045 0.053 — — — — 2-J0.048 0.91 2.73 0.04 0.006 0.036 0.002 0.033 0.185 0.155 0.13 0.13 0.120.11 2-K 0.042 0.82 2.78 0.04 0.009 0.035 0.002 0.008 — — — — — 2-L0.038 0.91 2.73 0.04 0.006 0.036 0.002 0.522 — — — — — 2-M 0.038 0.762.57 0.03 0.001 0.034 0.002 0.087  0.0005 — — — — — 2-N 0.039 0.76 2.550.03 0.001 0.035 0.002 0.032 0.056 — — — — — 2-O 0.042 0.73 2.49 0.030.001 0.036 0.002 0.092 0.382 — — — — — 2-P 0.043 0.75 2.52 0.03 0.0010.035 0.002 0.088 0.453 — — — — — 2-Q 0.041 0.70 2.55 0.03 0.002 0.0320.002 0.098 —  0.0005 — — — — 2-R 0.038 0.71 2.58 0.03 0.002 0.030 0.0020.025 — 0.037 — — — — 2-S 0.039 0.74 2.57 0.04 0.002 0.030 0.002 0.079 —0.186 — — — — 2-T 0.042 0.71 2.51 0.02 0.002 0.029 0.002 0.089 — 0.356 —— — — Transformation point Steel (° C.) No. X*¹ Y*² Z*³ A_(c1) A_(c3)Remarks 2-A 0.91 — — 707 842 Acceptable example 2-B — 1.37 1.51 718 868Acceptable example 2-C — 3.00 4.82 719 915 Acceptable example 2-D — 1.281.56 715 875 Acceptable example 2-E — 1.89 1.52 717 870 Acceptableexample 2-F — 7.60 1.47 718 972 Comparative example 2-G — 0.44 0.78 717851 Comparative example 2-H — 1.92 2.68 717 931 Acceptable example 2-I —1.70 1.15 717 900 Acceptable example 2-J — 2.77 16.13  722 932Acceptable example 2-K 0.04 — — 717 866 Comparative example 2-L 3.23 — —720 923 Comparative example 2-M — 0.54 0.01 718 868 Acceptable example2-N — 0.56 1.92 718 868 Acceptable example 2-O — 2.86 4.55 718 868Acceptable example 2-P — 3.20 5.65 718 868 Comparative example 2-Q —0.57 0.01 715 875 Acceptable example 2-R — 0.64 3.15 715 875 Acceptableexample 2-S — 2.86 5.00 715 875 Acceptable example 2-T — 4.74 8.50 715875 Comparative example

[0129] TABLE 3(a) Steel Chemical composition (mass %) No. C Si Mn P S AlN V Nb Ti Cr Mo Cu Ni 3-A 0.028 0.02 1.55 0.01 0.003 0.034 0.002 0.121 —— — — — — 3-B 0.030 0.02 1.46 0.01 0.002 0.035 0.002 0.108 0.041 — —0.16 — — 3-C 0.031 0.03 1.70 0.01 0.005 0.028 0.002 0.086 0.036 0.0330.06 — — — 3-D 0.021 0.02 1.65 0.01 0.005 0.034 0.002 0.068 — — — — 0.140.07 3-E 0.032 0.02 1.52 0.01 0.004 0.033 0.002 0.124 0.044 — — 0.15 — —3-F 0.026 0.02 1.52 0.01 0.003 0.035 0.002 0.122 0.112 0.122 — — — — 3-G0.032 0.02 1.62 0.01 0.005 0.032 0.002 0.042 — — — — — — 3-H 0.021 0.212.02 0.06 0.003 0.030 0.002 0.130 — — — — — — 3-I 0.024 0.52 1.88 0.040.001 0.032 0.002 0.105 0.033 — 0.16 — — 3-J 0.026 0.32 1.72 0.01 0.0040.026 0.002 0.088 0.035 0.032 0.08 — — — 3-K 0.020 0.70 1.55 0.01 0.0030.028 0.002 0.073 0.045 — — — — — 3-L 0.012 0.21 1.51 0.01 0.002 0.0330.002 0.055 — 0.018 — — — — 3-M 0.018 0.50 1.56 0.03 0.004 0.035 0.0020.108 — — 0.05 0.15 — — Transformation point Steel (° C.) No. X*¹ Y*²Z*³ A_(c1) A_(c3) Remarks 3-A 1.02 — — 725 860 Acceptable example 3-B —1.20 0.42 705 855 Acceptable example 3-C — 1.48 1.27 710 850 Acceptableexample 3-D 0.76 — — 715 855 Acceptable example 3-E — 1.27 0.39 705 855Acceptable example 3-F — 4.56 3.13 725 855 Comparative example 3-G 0.31— — 715 850 Comparative example 3-H 1.46 — — 725 860 Acceptable example3-I — 1.38 0.34 705 860 Acceptable example 3-J — 1.76 1.21 710 860Acceptable example 3-K — 1.44 0.68 715 870 Acceptable example 3-L — 1.830.70 710 865 Acceptable example 3-M 1.41 — — 710 860 Acceptable example

[0130] TABLE 3(b) Steel Chemical composition (mass %) No. C Si Mn P S AlN V Nb Ti Cr Mo Cu Ni 3-N 0.020 0.39 1.73 0.05 0.001 0.031 0.002 0.110 —— 0.05 0.15 0.15 0.15 3-O 0.021 0.28 1.95 0.02 0.005 0.029 0.002 0.0750.038 0.01 0.05 0.15 0.10 0.10 3-P 0.008 0.32 1.75 0.01 0.005 0.0320.002 0.075 — — — — — — 3-Q 0.095 0.34 1.73 0.01 0.003 0.029 0.002 0.361— — — — — — 3-R 0.023 0.49 1.54 0.01 0.002 0.030 0.002 0.007 — — — — — —3-S 0.024 0.51 1.47 0.03 0.001 0.031 0.002 0.597 — — — — — — 3-T 0.0220.35 1.75 0.05 0.003 0.029 0.002 0.109  0.0005 — — — — — 3-U 0.023 0.441.78 0.04 0.003 0.027 0.002 0.065 0.319 — — — — — 3-V 0.021 0.35 1.730.05 0.001 0.034 0.002 0.099 —  0.0005 — — — — 3-W 0.025 0.36 1.77 0.050.002 0.032 0.002 0.132 — 0.321 — — — — 3-X 0.020 0.02 1.51 0.01 0.0030.033 0.002 0.035 0.001 0.001 — — — — 3-Y 0.023 0.02 1.66 0.01 0.0030.035 0.002 0.308 — — — — — — Transformation point Steel (° C.) No. X*¹Y*² Z*³ A_(c1) A_(c3) Remarks 3-N 1.29 — — 705 865 Acceptable example3-O — 1.55 0.84 715 865 Acceptable example 3-P 2.21 — — 714 881Comparative example 3-Q 0.89 — — 714 872 Comparative example 3-R 0.07 —— 722 874 Comparative example 3-S 5.85 — — 721 949 Comparative example3-T — 1.17 0.01 714 906 Acceptable example 3-U — 4.24 5.38 717 891Comparative example 3-V — 1.12 0.01 715 913 Acceptable example 3-W —7.66 5.17 715 1031 Comparative example 3-X — 0.45 0.09 707 866Comparative example 3-Y 3.15 — — 707 882 Comparative example

[0131] TABLE 4 Steel Chemical composition (mass %) No. C Si Mn P S Al NV Nb Ti Cr Mo Cu Ni 4-A 0.038 0.48 2.88 0.01 0.004 0.033 0.002 0.158 — —— — — — 4-B 0.041 0.77 2.51 0.01 0.001 0.035 0.002 0.056 0.171 — — 0.31— — 4-C 0.040 0.76 2.49 0.01 0.007 0.034 0.002 0.068 0.120 0.125 0.09 —— — 4-D 0.038 0.72 2.54 0.01 0.009 0.033 0.002 0.085 — 0.058 — — 0.080.07 4-E 0.049 0.74 2.53 0.01 0.006 0.036 0.002 0.039 0.075 0.005 — 0.31— — 4-F 0.039 0.75 2.55 0.01 0.007 0.035 0.002 0.183 0.191 0.260 — — — —4-G 0.046 0.73 2.57 0.01 0.007 0.038 0.002 0.011 0.013 0.015 — — — — 4-H0.039 0.93 2.95 0.05 0.004 0.039 0.002 0.016 0.003 0.108 — 0.27 — — 4-I0.041 0.80 2.80 0.05 0.009 0.033 0.002 0.138 0.042 0.065 — — — — 4-J0.047 0.92 2.78 0.04 0.006 0.034 0.002 0.025 0.175 0.143 0.15 — — — 4-K0.043 0.84 2.76 0.04 0.007 0.034 0.002 0.007 — — — — — 4-L 0.038 0.932.75 0.04 0.006 0.035 0.002 0.553 — — — — — 4-M 0.042 0.80 2.65 0.020.003 0.031 0.002 0.096  0.0005 — — — — — 4-N 0.041 0.81 2.68 0.02 0.0030.030 0.002 0.029 0.065 — — — — — 4-O 0.043 0.78 2.67 0.03 0.003 0.0290.002 0.087 0.295 — — — — — 4-P 0.041 0.77 2.69 0.02 0.002 0.030 0.0020.079 0.521 — — — — — 4-Q 0.039 0.76 2.74 0.03 0.003 0.031 0.002 0.105 — 0.0005 — — — — 4-R 0.043 0.79 2.74 0.02 0.004 0.033 0.002 0.035 — 0.042— — — — 4-S 0.040 0.80 2.75 0.03 0.002 0.032 0.002 0.087 — 0.182 — — — —4-T 0.038 0.81 2.77 0.02 0.003 0.032 0.002 0.089 — 0.290 — — — —Transformation point Steel (° C.) No. X*¹ Y*² Z*³ A_(c1) A_(c3) Remarks4-A 0.98 — — 706 842 Acceptable example 4-B — 1.40 3.35 719 865Acceptable example 4-C — 2.74 5.84 720 905 Acceptable example 4-D — 1.291.45 716 876 Acceptable example 4-E — 0.63 2.38 717 859 Acceptableexample 4-F — 5.70 4.16 718 971 Comparative example 4-G — 0.29 4.19 717851 Comparative example 4-H — 1.50 14.55 717 922 Acceptable example 4-I— 1.85 1.33 716 909 Acceptable example 4-J — 2.61 19.83 723 923Acceptable example 4-K 0.04 — — 718 867 Comparative example 4-L 3.42 — —721 924 Comparative example 4-M — 0.54 0.01 719 865 Acceptable example4-N — 0.58 2.46 719 865 Acceptable example 4-O — 2.25 3.72 719 865Acceptable example 4-P — 3.73 7.23 719 865 Comparative example 4-Q —0.64 0.01 716 876 Acceptable example 4-R — 0.68 2.55 716 876 Acceptableexample 4-S — 2.79 4.45 716 876 Acceptable example 4-T — 4.37 6.92 716876 Comparative example

[0132] TABLE 5(a) Cold rolling Annealing Microstructure Mechanicalproperties of temperature in Ferrite Second phase cold rolled steelsheet Steel continuous phase Area ratio of Area ratio Tensile propertiessheet Steel annealing line Area martensite of second YS TS El YR No. No.(° C.) ratio (%) Kind*¹ (%) phase (%) (MPa) (MPa) (%) (%) r-valueRemarks 1 1-A 830 92 M 8 8 330 600 31 55 1.8 Invention example 2 1-B 83090 M 10  10 330 610 30 54 1.8 Invention example 3 1-B 980 0 P, B, M 15 100 650 720 22 90 0.9 Comparative example 4 1-B 680 100 — 0 0 450 530 2985 0.8 Comparative Example 5 1-C 830 92 M 8 8 340 600 31 57 1.8Invention example 6 1-D 830 90 M 10  10 330 610 30 54 1.4 Inventionexample 7 1-E 830 92 M 8 8 310 570 33 54 1.7 Invention example 8 1-F 830100 — 0 0 510 600 27 85 1.8 Comparative example 9 1-G 830 93 M 7 7 330610 31 54 0.8 Comparative example 10 1-H 850 92 M 8 8 350 630 29 56 1.9Invention example 11 1-I 850 93 M 7 7 330 620 30 53 1.9 Inventionexample 12 1-J 850 92 M 8 8 330 610 33 54 1.8 Invention example 13 1-K830 92 M 8 8 245 450 38 54 1.9 Invention example 14 1-L 830 93 M 7 7 330605 30 55 1.8 Invention example

[0133] TABLE 5(b) Cold rolling Annealing Microstructure Mechanicalproperties of temperature in Ferrite Second phase cold rolled steelsheet Steel continuous phase Area ratio of Area ratio Tensile propertiessheet Steel annealing line Area martensite of second YS TS El YR No. No.(° C.) ratio (%) Kind*¹ (%) phase (%) (MPa) (MPa) (%) (%) r-valueRemarks 15 1-M 830 92 M 8 8 340 620 30 55 1.7 Invention example 16 1-N830 93 M 7 7 320 600 31 53 1.7 Invention example 17 1-O 830 92 M, B 6 8340 625 29 54 1.8 Invention example 18 1-P 830 100 — 0 0 425 520 34 821.9 Comparative Example 19 1-Q 830 65 M 35 35  395 670 29 59 0.8Comparative example 20 1-R 850 69 M 31 31  370 620 30 60 0.8 Comparativeexample 21 1-S 850 100 — 0 0 495 615 30 80 1.7 Comparative example 221-T 850 92 M 8 8 355 575 32 62 1.7 Invention example 23 1-U 850 100 — 00 470 580 31 81 1.8 Comparative example 24 1-V 830 91 M 9 9 350 570 3261 1.7 Invention example 25 1-W 850 100 — 0 0 480 595 31 81 1.8Comparative example 26 1-X 830 72 M 28 28  350 560 31 63 0.8 Comparativeexample 27 1-Y 830 100 — 0 0 475 590 30 81 1.7 Comparative example

[0134] TABLE 6(a) Cold rolling Annealing Microstructure Mechanicalproperties of temperature in Ferrite Second phase cold rolled steelsheet Steel continuous phase Area ratio of Area ratio Tensile propertiessheet Steel annealing line Area martensite of second YS TS El YR No. No.(° C.) ratio (%) Kind*¹ (%) phase (%) (MPa) (MPa) (%) (%) r-valueRemarks 28 2-A 780 90 M 10 10 560 825 19 68 1.1 Invention example 29 2-B780 87 M 13 13 550 810 19 68 1.3 Invention example 30 2-B 950 0 P, B, M19 100 740 860 16 86 0.7 Comparative example 31 2-B 680 100 —  0 0 625770 22 81 0.8 Comparative Example 32 2-C 750 88 M 12 12 540 805 20 671.3 Invention example 33 2-D 760 88 M 12 12 545 810 19 67 1.2 Inventionexample 34 2-E 770 87 M 13 13 550 820 20 67 1.3 Invention example 35 2-F780 100 —  0 0 660 830 19 80 1.4 Comparative example 36 2-G 780 69 M 3131 540 820 20 66 0.7 Comparative example 37 2-H 760 81 M 19 19 620 93015 67 1.3 Invention example 38 2-I 780 83 M 17 17 590 860 17 69 1.1Invention example

[0135] TABLE 6(b) Cold rolling Annealing Microstructure Mechanicalproperties of temperature in Ferrite Second phase cold rolled steelsheet Steel continuous phase Area ratio of Area ratio Tensile propertiessheet Steel annealing line Area martensite of second YS TS El YR No. No.(° C.) ratio (%) Kind*¹ (%) phase (%) (MPa) (MPa) (%) (%) r-valueRemarks 39 2-J 780 87 M 13 13 445 660 27 67 1.4 Invention example 40 2-K760 68 M 32 32 570 850 18 67 0.8 Comparative example 41 2-L 780 100 —  00 690 835 19 83 1.3 Comparative example 42 2-M 780 85 M 15 15 525 805 2065 1.1 Invention example 43 2-N 760 88 M 12 12 530 800 20 66 1.3Invention example 44 2-O 780 90 M 10 10 525 790 21 66 1.3 Inventionexample 45 2-P 780 100 —  0 0 650 795 21 82 1.3 Comparative example 462-Q 760 87 M 13 13 540 810 19 67 1.1 Invention example 47 2-R 760 88 M12 12 545 815 15 67 1.3 Invention example 48 2-S 780 90 M 10 10 540 81019 67 1.3 Invention example 49 2-T 780 100 —  0 0 665 785 20 85 1.4Comparative example

[0136] TABLE 7(a) Galvanizing Annealing Microstructure Mechanicalproperties of temperature in Ferrite Second phase galvanized steel sheetSteel continuous phase Area ratio of Area ratio Tensile properties sheetSteel annealing line Area martensite of second YS TS El YR No. No. (°C.) ratio (%) Kind*¹ (%) phase (%) (MPa) (MPa) (%) (%) r-value Remarks50 3-A 830 92 M 8 8 330 610 31 54 1.7 Invention example 51 3-B 830 90 M10  10 330 620 30 53 1.7 Invention example 52 3-B 830 92 M 8 8 350 63030 56 1.6 Invention example 53 3-B 980 0 P, B, M 12  100 660 720 22 920.9 Comparative Example 54 3-B 680 100 — 0 0 460 540 28 85 0.8Comparative example 55 3-C 830 90 M 10  10 340 610 31 56 1.7 Inventionexample 56 3-D 830 92 M 8 8 340 620 30 55 1.4 Invention example 57 3-E830 94 M 6 6 320 580 32 55 1.6 Invention example 58 3-F 830 100 — 0 0510 600 27 85 1.7 Comparative example 59 3-G 830 92 M 8 8 330 610 30 540.8 Comparative example 60 3-H 850 93 M 7 7 340 630 30 54 1.8 Inventionexample 61 3-I 850 92 M 8 8 340 620 31 55 1.8 Invention example 62 3-J850 92 M 8 8 320 610 31 52 1.7 Invention example 63 3-K 830 92 M, B 6 8330 610 30 54 1.6 Invention example 64 3-L 830 92 M 8 8 248 450 37 551.7 Invention example 65 3-M 830 93 M 7 7 340 620 30 55 1.6 Inventionexample

[0137] TABLE 7(b) Galvanizing Annealing Microstructure Mechanicalproperties of temperature in Ferrite Second phase galvanized steel sheetSteel continuous phase Area ratio of Area ratio Tensile properties sheetSteel annealing line Area martensite of second YS TS El YR No. No. (°C.) ratio (%) Kind*¹ (%) phase (%) (MPa) (MPa) (%) (%) r-value Remarks66 3-N 830 92 M 8 8 320 600 31 53 1.6 Invention example 67 3-O 830 93 M7 7 340 625 29 54 1.7 Invention example 68 3-H 830 92 M 8 8 340 620 3055 1.8 Invention example 69 3-K 830 93 M 7 7 320 600 31 53 1.6 Inventionexample 70 3-M 830 92 M 8 8 320 610 31 52 1.6 Invention example 71 3-P830 100 — 0 0 420 510 34 82 1.8 Comparative example 72 3-Q 830 66 M 34 34 390 670 27 58 0.8 Comparative example 73 3-R 850 68 M 32  32 385 61530 63 0.8 Comparative example 74 3-S 850 100 — 0 0 500 605 31 83 1.6Comparative example 75 3-T 850 91 M 9 9 350 580 31 60 1.7 Inventionexample 76 3-U 850 100 — 0 0 480 575 32 83 1.6 Comparative example 773-V 830 91 M 9 9 340 580 31 59 1.7 Invention example 78 3-W 850 100 — 00 490 600 30 82 1.7 Comparative example 79 3-X 830 70 M 30  30 340 56532 60 0.8 Comparative example 80 3-Y 830 100 — 0 0 490 600 30 82 1.7Comparative example

[0138] TABLE 8(a) Galvanizing Annealing Microstructure Mechanicalproperties of temperature in Ferrite Second phase galvanized steel sheetSteel continuous phase Area ratio of Area ratio Tensile properties sheetSteel annealing line Area martensite of second YS TS El YR No. No. (°C.) ratio (%) Kind*¹ (%) phase (%) (MPa) (MPa) (%) (%) r-value Remarks81 4-A 780 91 M  9 9 560 815 19 69 1.1 Invention example 82 4-B 780 89 M11 11 555 805 19 69 1.4 Invention example 83 4-B 950 0 P,B,M 21 100 735850 16 86 0.8 Comparative example 84 4-B 680 100 —  0 0 620 760 22 820.8 Comparative Example 85 4-C 4 89 M 11 11 545 800 20 68 1.3 Inventionexample 86 4-D 760 88 M 12 12 550 805 19 68 1.4 Invention example 87 4-E770 90 M 10 10 550 810 20 68 1.3 Invention example 88 4-F 780 100 —  0 0675 815 19 83 1.5 Comparative example 89 4-G 780 92 M  8 8 550 810 20 680.8 Comparative example 90 4-H 760 83 M 17 17 635 935 15 68 1.3Invention example 91 4-I 780 85 M 15 15 590 855 17 69 1.1 Inventionexample

[0139] TABLE 8(b) Galvanizing Annealing Microstructure Mechanicalproperties of temperature in Ferrite Second phase galvanized steel sheetSteel continuous phase Area ratio of Area ratio Tensile properties sheetSteel annealing line Area martensite of second YS TS El YR No. No. (°C.) ratio (%) Kind*¹ (%) phase (%) (MPa) (MPa) (%) (%) r-value Remarks92 4-J 780 85 M 15 15 440 665 25 68 1.4 Invention example 93 4-K 760 67M 33 33 560 860 18 65 0.8 Comparative example 94 4-L 780 100 —  0 0 695840 19 83 1.4 Comparative example 95 4-M 780 86 M 14 14 510 810 20 631.1 Invention example 96 4-N 760 89 M 11 11 525 800 20 66 1.3 Inventionexample 97 4-O 780 89 M 11 11 525 795 20 66 1.3 Invention example 98 4-P780 100 —  0 0 660 805 20 82 1.4 Comparative example 99 4-Q 760 87 M 1313 525 810 19 65 1.1 Invention example 100 4-R 760 86 M 14 14 530 810 1965 1.2 Invention example 101 4-S 780 89 M 11 11 540 820 18 66 1.3Invention example 102 4-T 780 100 —  0 0 660 790 20 84 1.3 Comparativeexample

[0140] As seen from the results shown in Tables 5 and 6, the cold rolledsteel sheets in all invention examples have a low yield stress (YS), ahigh elongation (E1) and a low yield ratio (YR) and further indicate ahigh r-value and are excellent in the deep drawability, and have atensile strength (TS) of not less than 440 MPa. On the contrary, in thecomparative examples being outside the range of the invention, the yieldstress (YS) is high, the elongation (E1) is low, or the r-value is low.Particularly, the somewhat lowering of the r-value accompanied with thehigh-strengthening is observed in the high-strength steel sheets of TS:not less than 780 MPa shown in Table 6, for example, the steel sheet No.28 produced by using the steel No. 2-A containing V and no Nb and Ti andthe steel sheet No. 38 produced by using the steel No. 2-I containing V,Nb and Ti and satisfying a relationship of0.5×C/12≦(V/51+2×Nb/93+2×Ti/48)≦3×C/12 but satisfying a relationship of(2×Nb/93+2×Ti/48)/(V/51)<0.5. On the other hand, the r-value is improvedin the steel sheet Nos. 29, 32, 33 and 34 produced by using the steelNos. 2-B, 2-C, 2-D and 2-E containing V, Nb and Ti and satisfying bothrelationships of 0.5×C/12≦(V/51+2×Nb/93+2×Ti/48)≦3×C/12 and1.5≦(2×Nb/93+2×Ti/48)/(V/51)≦15.

[0141] And also, the results obtained with respect to the galvanizedsteel sheets are shown in Tables 7 and 8. Even in these galvanized steelsheets, the results similar to those of the above cold rolled steelsheets are obtained.

[0142] In the steel sheet according to the invention, excellentproperties are obtained even by the production process conducting thegalvanization.

[0143] Industrial Applicability

[0144] The invention develops an industrially remarkable effect that thehigh-strength cold rolled steel sheet and galvanized steel sheet havingan excellent deep drawability can be produced stably. When the coldrolled steel sheet and the galvanized steel sheet according to theinvention are applied to vehicle parts, there are effects that the pressforming is easy and they can sufficiently contribute to reduce theweight of the vehicle body.

1. A high-strength dual-phase cold rolled steel sheet having anexcellent deep drawability, characterized in that the steel sheet has acomposition comprising C: 0.01-0.08 mass %, Si: not more than 2.0 mass%, Mn: not more than 3.0 mass %, P: not more than 0.10 mass %, S: notmore than 0.02 mass %, Al: 0.005-0.20 mass %, N: not more than 0.02 mass% and V: 0.01-0.5 mass %, provided that V and C satisfy a relationshipof 0.5×C/12≦V/51≦3×C/12, and the remainder being Fe and inevitableimpurities, and has a microstructure consisting of a ferrite phase as aprimary phase and a secondary phase including martensite phase at anarea ratio of not less than 1% to a whole of the microstructure.
 2. Ahigh-strength dual-phase cold rolled steel sheet having an excellentdeep drawability, characterized in that the steel sheet has acomposition comprising C: 0.01-0.08 mass %, Si: not more than 2.0 mass%, Mn: not more than 3.0 mass %, P: not more than 0.10 mass %, S: notmore than 0.02 mass %, Al: 0.005-0.20 mass %, N: not more than 0.02 mass% and V: 0.01-0.5 mass % and further comprising not more than 0.3 mass %in total of one or two of Nb: more than 0 mass % but not more than 0.3mass % and Ti: more than 0 mass % but not more than 0.3 mass %, providedthat V, Nb, Ti and C satisfy a relationship of0.5×C/12≦(V/51+2×Nb/93+2×Ti/48)≦3×C/12, and the remainder being Fe andinevitable impurities, and has a microstructure consisting of a ferritephase as a primary phase and a secondary phase including martensitephase at an area ratio of not less than 1% to a whole of themicrostructure.
 3. A high-strength dual-phase cold rolled steel sheethaving an excellent deep drawability according to claim 2, wherein thesteel sheet comprises not more than 0.3 mass % in total of one or two ofNb: 0.001-0.3 mass % and Ti: 0.001-0.3 mass %.
 4. A high-strengthdual-phase cold rolled steel sheet having an excellent deep drawabilityaccording to claim 2, wherein the steel sheet comprises C: 0.03-0.08mass %, Si: 0.1-2.0 mass %, Mn: 1.0-3.0 mass %, P: not more than 0.05mass % and S: not more than 0.01 mass %, provided that V, Nb and Tisatisfy a relationship of 1.5≦(2×Nb/93+2×Ti/48)/(V/51)≦15.
 5. Ahigh-strength dual-phase cold rolled steel sheet having an excellentdeep drawability according to any one of claims 1-4, wherein the steelsheet further comprises one or two of the following A-group and B-group:A-group: not more than 2.0 mass % in total of one or two of Cr and Mo;B-group: not more than 2.0 mass % in total of one or two of Cu and Ni.6. A method of producing a high-strength dual-phase cold rolled steelsheet having an excellent deep drawability, which comprises hot rollinga steel slab having a composition comprising C: 0.01-0.08 mass %, Si:not more than 2.0 mass %, Mn: not more than 3.0 mass %, P: not more than0.10 mass %, S: not more than 0.02 mass %, Al: 0.005-0.20 mass %, N: notmore than 0.02 mass % and V: 0.01-0.5 mass %, provided that V and Csatisfy a relationship of 0.5×C/12≦V/51≦3×C/12, and the remainder beingFe and inevitable impurities, pickling, cold rolling and then subjectingto a continuous annealing at a temperature range from a A_(C1)transformation point to a A_(C3) transformation point.
 7. A method ofproducing a high-strength dual-phase cold rolled steel sheet having anexcellent deep drawability, which comprises hot rolling a steel slabhaving a composition comprising C: 0.01-0.08 mass %, Si: not more than2.0 mass %, Mn: not more than 3.0 mass %, P: not more than 0.10 mass %,S: not more than 0.02 mass %, Al: 0.005-0.20 mass %, N: not more than0.02 mass % and V: 0.01-0.5 mass % and further comprising not more than0.3 mass % in total of one or two of Nb: more than 0 mass % but not morethan 0.3 mass % and Ti: more than 0 mass % but not more than 0.3 mass %,provided that V, Nb, Ti and C satisfy a relationship of0.5×C/12≦(V/51+2×Nb/93+2×Ti/48)≦3×C/12, and the remainder being Fe andinevitable impurities, pickling, cold rolling and then subjecting to acontinuous annealing at a temperature range from a A_(C1) transformationpoint to a A_(C3) transformation point.
 8. A method of producing ahigh-strength dual-phase cold rolled steel sheet having an excellentdeep drawability according to claim 7, wherein the steel slab comprisesnot more than 0.3 mass % in total of one or two of Nb: 0.001-0.3 mass %and Ti: 0.001-0.3 mass %.
 9. A method of producing a high-strengthdual-phase cold rolled steel sheet having an excellent deep drawabilityaccording to claim 7, wherein the steel slab comprises C: 0.03-0.08 mass%, Si: 0.1-2.0 mass %, Mn: 1.0-3.0 mass %, P: not more than 0.05 mass %and S: not more than 0.01 mass %, provided that V, Nb and Ti satisfy arelationship of 1.5≦(2×Nb/93+2×Ti/48)/(V/51)≦15.
 10. A method ofproducing a high-strength dual-phase cold rolled steel sheet having anexcellent deep drawability according to any one of claims 6-9, whereinthe steel slab further comprises one or two of the following A-group andB-group: A-group: not more than 2.0 mass % in total of one or two of Crand Mo; B-group: not more than 2.0 mass % in total of one or two of Cuand Ni.
 11. A high-strength dual-phase galvanized steel sheet having anexcellent deep drawability comprising a galvanized coating on the steelsheet as claimed in claim
 1. 12. A high-strength dual-phase galvanizedsteel sheet having an excellent deep drawability comprising a galvanizedcoating on the steel sheet as claimed in claim
 2. 13. A high-strengthdual-phase galvanized steel sheet having an excellent deep drawabilitycomprising a galvanized coating on the steel sheet as claimed in claim3.
 14. A high-strength dual-phase galvanized steel sheet having anexcellent deep drawability comprising a galvanized coating on the steelsheet as claimed in claim
 4. 15. A high-strength dual-phase galvanizedsteel sheet having an excellent deep drawability comprising a galvanizedcoating on the steel sheet as claimed in claim
 5. 16. A method ofproducing a high-strength dual-phase galvanized steel sheet having anexcellent deep drawability, characterized by subjecting to agalvanization after the continuous annealing at a temperature range froma A_(C1) transformation point to a A_(C3) transformation point in themethod claimed in claim
 6. 17. A method of producing a high-strengthdual-phase galvanized steel sheet having an excellent deep drawabilityaccording to claim 16, characterized by further comprising a continuousannealing step between the cold rolling step and the continuousannealing step at a temperature range from a A_(C1) transformation pointto a A_(C3) transformation point.
 18. A method of producing ahigh-strength dual-phase galvanized steel sheet having an excellent deepdrawability, characterized by subjecting to a galvanization after thecontinuous annealing at a temperature range from a A_(C1) transformationpoint to a A_(C3) transformation point in the method claimed in claim 7.19. A method of producing a high-strength dual-phase galvanized steelsheet having an excellent deep drawability according to claim 18,characterized by further comprising a continuous annealing step betweenthe cold rolling step and the continuous annealing step at a temperaturerange from a A_(C1) transformation point to a A_(C3) transformationpoint.
 20. A method of producing a high-strength dual-phase galvanizedsteel sheet having an excellent deep drawability, characterized bysubjecting to a galvanization after the continuous annealing at atemperature range from a A_(C1) transformation point to a A_(C3)transformation point in the method claimed in claim
 8. 21. A method ofproducing a high-strength dual-phase galvanized steel sheet having anexcellent deep drawability according to claim 20, characterized byfurther comprising a continuous annealing step between the cold rollingstep and the continuous annealing step at a temperature range from aA_(C1) transformation point to a A_(C3) transformation point.
 22. Amethod of producing a high-strength dual-phase galvanized steel sheethaving an excellent deep drawability, characterized by subjecting to agalvanization after the continuous annealing at a temperature range froma A_(C1) transformation point to a A_(C3) transformation point in themethod claimed in claim
 9. 23. A method of producing a high-strengthdual-phase galvanized steel sheet having an excellent deep drawabilityaccording to claim 22, characterized by further comprising a continuousannealing step between the cold rolling step and the continuousannealing step at a temperature range from a A_(C1) transformation pointto a A_(C3) transformation point.
 24. A method of producing ahigh-strength dual-phase galvanized steel sheet having an excellent deepdrawability according to any one of claims 16-23, wherein the steel slabfurther comprises one or two of the following A-group and B-group:A-group: not more than 2.0 mass % in total of one or two of Cr and Mo;B-group: not more than 2.0 mass % in total of one or two of Cu and Ni.